RIO DE JANEIRO — Massive protests calling for the impeachment of Brazilian President Dilma Rousseff have narrowed her options to fend off political and economic crises and will mean more turbulence for the nation in the months to come, but her ouster remains highly unlikely, analysts said Monday.

Polls showed the Rousseff government's popularity had already been cut in half before Sunday's protests, compared to when she won re-election in October, and the sheer size of the challenge facing her was splashed across front pages of newspapers Monday with photos of the surprisingly huge crowds seeking her removal.

Anger over a sprawling corruption scheme sparked Sunday's marches in more than 150 cities.

Federal prosecutors say they've uncovered Brazil's biggest graft case yet in a kickback scheme at state-run oil company Petrobras, with at least $800 million paid by construction and engineering firms in bribes to politically appointed former executives at the oil company, all in exchange for winning inflated contracts.

Investigators say some of the money was funneled back to the campaign coffers of the Workers' Party and its allies. Dozens of congressmen and some former executive branch officials, including two former chiefs of staff to Rousseff, are under investigation. The president, who served as chairwoman of Petrobras' board during several years as the graft took place, isn't implicated, but polls show a large majority of Brazilians think she knew about the pilfering.

Some 210,000 people gathered on a main Sao Paulo avenue Sunday, a crowd larger than any seen during daily anti-government rallies in June 2013, when anger was less focused on Rousseff herself than on public anger over poor public services and perceived endemic political corruption in general.

The new protests are expected to be less frequent. The right-leaning groups who organized Sunday's events on social media have called for a new round on April 12.

In Brasilia on Monday, Rousseff said she would send anti-corruption measures to Congress this week and that she firmly backs political reforms, though she offered no details. The president, who as a leftist guerrilla was imprisoned for three years during Brazil's dictatorship and brutally tortured, underscored her personal history and her support of the right to protest because "many people from my generation gave their lives so that people could express themselves in the streets."

"Yesterday, when I saw hundreds of thousands of citizens demonstrating, I couldn't help but think that it was worth it, that fight for freedom, that it was worth it to fight for democracy," Rousseff said. "Our nation is stronger than ever."

Despite the anger expressed in the streets, few analysts expect Rousseff's departure.

"While the next few months will be very difficult, we still don't think the government is headed to a governability crisis or that Rousseff is likely to be impeached," the Eurasia Group political risk consulting firm wrote in a Monday note.

Eurasia emphasized that it's difficult to impeach a sitting president, and that "there are a number of examples to demonstrate presidents can survive very low approval ratings and very unpopular periods of governance." It cited a moment in 1999 when Brazil's currency was devalued and then-President Fernando Henrique Cardoso had single-digit poll numbers but remained in office.

Still, Sunday's wave of protests illustrated how far from grace the ruling Workers' Party has fallen since it took the presidency in 2003. Created in 1980 by a collection of strong unions that fought Brazil's long dictatorship and a wide range of non-government organizations representing a wide cross-section of Brazilian society, the party has always been known for its ability to rally its adherents into the streets.

Not so now. Pro-Rousseff demonstrations that put thousands in the streets last Friday were tepid events orchestrated by unions and social groups, lacking the energy and spontaneity seen Sunday.

"Who could have imagined that the Workers' Party would flip sides and now be the target, after 30 years of glories," wrote Eliane Cantanhede, a political columnist for the Estado de S. Paulo newspaper.

She said the government needs stronger efforts to work with the opposition and push through congressional measures to spark Brazil's moribund economy and make substantial political reforms.

Political commentator Merval Pereira wrote in Monday's O Globo newspaper that if Brazil had a parliamentary system, "the government would have fallen by now," noting that even Rousseff's allies in Congress, many of whom are under investigation in the Petrobras graft case, are blocking reform measures she has introduced.

"To recover her political legitimacy, she would have to reinvent herself, she would have to become a different Dilma, which appears impossible," Pereira wrote. "But she'll continue to govern ... amid political and economic crises that are increasingly severe, until the presidential election of 2018 allows and alternation of power."